For various industrial processes it is frequently necessary to move or press, or both, materials which are corrosive or abrasive and this often must be accomplished at relatively high temperatures. Extrusion machines are an example wherein the most commonly used machines are single screw or worm shaft extruders. Material is normally added in granular or powder form via a hopper and is moved by a coarse-pitched archimedean screw rotating in a barrel which may be electrically heated. Such archimedean screws are considered to have three sections. The first section is a feed section which conveys solid material from wherever it is being fed into the machine and here the screw has a channel depth that is generally uniform and relatively large. Subsequent compression occurs in a compression section which has reducing channel depth and is intended to compact and force the material into contact through the barrel to encourage heating. Melting may be achieved by a combination of heat conducted from the barrel and heat generated by the shearing the molten layer formed between the barrel and the solid material. There is also a third metering section in which the channel depth of the screw is constant and relatively small. Its purpose is to control the output from the extruder in terms of quantity, steadiness and homogeneity. Further, the relative motion between the screw and the barrel creates an efficient mixing action in the melted material and is also capable of generating necessary pressures for extrusion. Both the barrels and the screws are composed of a hardened and/or corrosive resistant material to minimize wear and corrosion. The screw threads, known as flights, may be arranged to be continuous, but need not be continuous for the conveying and compaction of the material to occur.
Heretofore, the flights, feed quills, worms, collars and intermediate rings for worm shaft assemblies have been manufactured by arc welding or casting a corrosive or abrasive resistant material such as cobalt onto a base or hub of steel or other suitable material. These parts are very expensive. They are difficult to weld successfully because of dissimilar metals. The corrosive or abrasive resistant material often cannot be successfully welded to the desired material for the hub. The relatively thin overlay deposited by the welding technique tends to wear more quickly, depending on the environment, and where it wears through to the base, the overlay tends to strip or peel. Because the weld is very expensive to repair, the component is usually disposed of and replaced, the cost of reclaiming the overlay being too great to justify salvage attempts.
In the casting process, the outer shell is first cast in the desired configuration, machined and then pressed onto the hub where it is welded in place. Although the wearlife of the component is increased, it is more expensive than the arc welded structure and its cost of salvage also tends to be uneconomical.
Assemblies of the components are used in a great variety of manufacturing processes in plastics, rubber, paper, meat rendering, and other industries, and for pelletizing, oil feed processes and the like. They may be used to provide pressures up to fifteen hundred pounds per square inch and the materials being processed may range from quite cold to relatively high temperatures such as 450.degree. F. The wear surface is usually cobalt, but may be other thin materials such as nickel and iron based alloys which are wear or corrosion resistant or both.